py3: use bytes[n:n + 1] to get bytes in templater._parsetemplate()

mercurial/templater.py

@@ -169,7 +169,7 b' def _parsetemplate(tmpl, start, stop, qu'
             parsed.append(('string', parser.unescapestr(tmpl[pos:stop])))
             pos = stop
             break
-        c = tmpl[n]
+        c = tmpl[n:n + 1]
         bs = (n - pos) - len(tmpl[pos:n].rstrip('\\'))
         if bs % 2 == 1:
             # escaped (e.g. '\{', '\\\{', but not '\\{')